Method of cleaning heat exchangers

ABSTRACT

A method for removing carbonaceous fouling from shell and tube heat exchangers used in hydrocarbon processes which uses as the solvent a highly aromatic hydrocarbon and utilizes heat from the process itself to accomplish the removal of the carbonaceous fouling material is disclosed. This method is particularly useful to maintain the efficiency of heat exchangers between scheduled shutdown of particular units for extensive preventive maintenance.

United States Patent [191 Callahan et al.

METHOD OF CLEANING HEAT EXCHANGERS Inventors: Joseph W. Callahan, Sulphur, La.;

Gene R. Laningham, Houston, Tex.

Assignee: Cesco, Inc., Houston, Tex.

Filed: May 23, 1973 Appl. No.: 363,279

US. Cl 208/48 R, 134/20, 134/39, 134/40 Int. Cl C23g 5/02, ClOg 9/16 Field of Search 208/48 R; 134/2, 20, 34, 134/40 References Cited UNITED STATES PATENTS 7/1945 Simpson 134/30 1 Nov. 26, 1974 2,953,514 9/1960 Wilkins 208/95 3,054,700 9/1962 Martin.... 134/2 3,060,064 10/1962 Zingg 134/2 Primary ExaminerDelbert E. Gantz Assistant Examiner-G. E. Schmitkons Attorney, Agent, or Firm-Prave1 & Wilson [5 7] ABSTRACT A method for removing carbonaceous fouling from shell and tube heat exchangers used in hydrocarbon processes which uses as the solvent a highly aromatic hydrocarbon and utilizes heat from the process itself to accomplish the removal of the: carbonaceous fouling material is disclosed. This method is particularly useful to maintain the efficiency of heat exchangers between scheduled shutdown of particular units for extensive preventive maintenance,

11 Claims, 1 Drawing Figure 1 METHOD OF CLEANING HEAT EXCHANGERS BACKGROUND OF THE INVENTION This invention relates to a method of cleaning shell and tube heat exchangers used in hydrocarbon processes where a cool stream flowing on one side of the heat exchanger is heated using the heat from another hotter process stream flowing on the other side of the heat exchanger in order to utilize all energy in the parti'cular process to its best advantage. It is the usual practicein the hydrocarbon processing industry, whether it be the refining of petroleum crude oil to usable products or the conversion of such products in petrochemical plants to further useful materials, to periodically shut down units for inspection and preventive maintenance. The periodic planned shutdown best utilizes the maintenance force of the particular plant involved and is also used to plan production rates to compensate for the period of time when a particular operating unit or plant is shut down for maintenance. In the interim be tween these planned shutdowns, heat exchangers in the hydrocarbon processes become fouled and the effi ciency of operating said exchangers decreases, causing a loss in energy which is either compensated for by consuming more energy through the use of heaters, and the like, or by suffering a decrease in production rate in the unit. ln either case the overall efficiency is decreased and energy is being wasted.

This loss of efficiency normally occurs just with the passage oftime due to the formation of deposits which, in the case of hydrocarbon processing in petroleum refineries and petrochemical plants, are generally carbonaceous and may be formed by deposition of suspended solid material or by the cracking and coking of the fluid streams.

Previously, the cleaning of these heat exchangers has required shutting down the unit or the heat exchanger, opening it and'removing the tube bundle inside, and then cleaning the tube side by either drilling, water blast, and more recently, by oxidation of the material using an oxygen-containing gas with heat '(see US. Pat. No. 3,054,700). The shell side of the tube bundles have been heretofore cleaned by water blasting, sand blasting, acid washing or flame treating while circulating coolant through the tubes (US. Pat. No. 3,060,064). All of the above-discussed treatments require excess amounts of down time and, hence; loss of energy and- /or production during the period occurs. It has also been attempted to use certain solvents for cleaning heat exchanger surfaces, as described in US. Pat. No. 2,380,340, but the practice using solvents has generally been disappointing since either several solvents must be sequentially used, the time involved of interrupting the process use has been excessive, or the degree of cleaning has not been effective.

SUMMARY OF THE INVENTION In accordance with this invention, therefore, an improved method for removing carbonaceous fouling from shell and tube heat exchangers used in hydrocarbon processes is described whereby the process stream on one side of the shell and tube heat exchanger is dis placed with a high-boiling hydrocarbon solvent having at least 50 wt. percent aromatic content, a Kauributanol value of at least about 90 and a minimum boiling point of at least about 350F and a maximum boiling point of at least about 500F. Regardless of the side of the heat exchanger to cleaned, whether it be the tube side or shell exchanger cleaned, whether it be the tube side or shell stand while the other side operates normally with the process stream flowing therethrough providing the heat which advantageously accelerates the cleaning process on the side of the exchanger being cleaned. In the practice of this invention, it is found that it is only necessary to continue the cleaning while holding the solvent solution in the shell and tube exchanger for a period of about 8 to about 24 hours to materially increase the heat transfer ofthe particular piece of equipment and to return it to an efficient operating range. After the cleaningtime is finished, the solvent which is itself a hydrocarbon can be either bled out of the heat exchanger by allowing the normal material to displace the solvent or it can be, in the case of where the process stream is a crude stream or a waste stream, pushed by said process stream into the process itself. This avoids any messy disposal problems for the solvent containing the removed carbonaceous fouling materials. Once the solvent and removed fouling material have been displaced from the side of the heat exchanger being cleaned, it is then returned to service in the process. The method of this invention is particularly well adaptable to cleaning the shell and tube heat exchangers in hydrocarbon processes such as the refining of petroleum products and the production of petrochemical materials. One particularly advantageous application of this invention is to clean the heat exchangers used to preheat crude oil prior to its being charged to fractionation towers. The crude is also further heated in an oven and delivered to fractionation towers with certain products from the fractionation tower being passed on the other side of the aforementioned heat exchanger to provide the heat for the incoming crude and also to cool the product from the tower. These crude preheat trains may be only a few heat exchangers or a bank of sometimes as many as fifty such heat exchangers, depending upon the size of the refinery. As the operation of the heat exchanger continues, whether the crude is passed on the shell side or the tube side ofthe heat exchanger, the exchanger begins to foul through the deposition of carbonaceous materials on the heat exchange surfaces. The heat transfer is then lowered and, to compensate for this inefficiency, the crude oven must consume more fuel to bring the crude stream to adequate temperature for efficient fractionation. This could lead to damage to the oven and, in any case, represents a waste of energy which could otherwise be conserved.

Another instance where the practice of this invention is particularly applicable is with respect to the preheat train to a catalytic reformer unit wherein the incoming feed is heated on one side of the exchanger by the passage of the hot reformate on the other side. While the reformate is very clean and does not cause fouling problems, the bottoms from the unit which are also used to heat the feed, can cause fouling problems. When the side containing a bottoms stream that is a waste stream is being cleaned, the BTU value in such a stream is lost. However, this loss is minimized by the fact that the heat exchanger is off stream only a minimum amount of time.

Those skilled in the art would readily recognize, from the above-described applications of heat exchangers which may be cleaned through the method of this invention, other such applications of the invention.

DESCRIPTION OF THE DRAWING The drawing attached hereto is a schematic diagram to aid in the explanation of applying the method of this invention to a shell and tube heat exchanger in service to conserve energy in hydrocarbon processing plants,

DESCRIPTION OF THE PREFERRED EMBODIMENT The heat exchangers in hydrocarbon processing plants, i.e., petroleum refineries and petrochemical plants, for reasons of economy, perform double duty. That is to say, a hot stream which needs to be cooled is used to supply heat in a shell and tube heat exchanger to a cooler stream which needs to be heated. In most cases, the shell and tube heat exchangers will be found to be employed in series or parallel, such that one or more of the exchangers may be taken out of service for one reason or another without causing the entire unit to become shut down. Often it would be found that several heat exchangers would be operated in series where the whole series would be in parallel with other similarly disposed and aligned heat exchangers. It is one of the particular advantages of the method of this invention that any down time of heat exchangers is minimized, thus effecting a significant savings in not only energy, but also in production of the hydrocarbon processing unit which is involved, since unit operation is disturbed when an exchanger is out of service.

As the heat exchangers are operated, carbonaceous material becomes attached to the heating surface and fouling occurs reducing the effectiveness of the heat transfer between the heating side and the cooling side, thus reducing the efficiency of the heat transfer. This heat transfer efficiency, generally designated as the heat transfer coefficient, U, BTU/(hour)(ft. )(F), can deteriorate to such a point that the overall efficiency of the unit is jeopardized. It is at this point that the invention described herein is especially important, since it allows the minimum of down time and the continuation of operation ofthe side of the heat exchanger not being cleaned.

In the operation of the method of this invention, the process stream utilizing the side of the exchanger being cleaned is displaced with the solvent used in the practice of the invention. Of course, the intake feed valve would be closed to stop the flow of material through the side being cleaned and the solvent would be retained in the heat exchanger in contact with the carbonaceous fouling material and cook under the influence of the material passing through the other side of the heat exchanger. This can be accomplished through either shutting down the heat exchanger and allowing it to cool before filling with the solvent used in the practice ofthis invention and reintroducing the process stream to the other side of the heat exchanger, or it can be accomplished by simply closing the feed valve to the side to be cleaned and pumping the solvent into the exchanger against the pressure of the outlet side ofthe exchanger to displace the feed material in the exchanger. Once the exchanger is full, then pumping is ceased and the action of the solvent on the carbonaceous fouling takes place. During this time, the outlet valve remains open and the exchanger floats" against the pressure in the exit line. To put the exchanger back on stream, the inlet feed valve is merely opened and the solvent flushed from the exchanger into the process stream. Of

course, if the solvent involved were detrimental to the purity of the stream going through the side of the exchanger being cleaned, then the outlet valve of the exchanger would be closed and the solvent removed through a bleed valve to prevent the contamination of the feed stream. However, in the case of cleaning the exchangers for the crude preheat train or the bottoms coolers of certain refinery columns, the process of this invention has particular application since the-solvent can usually be flushed into the stream without causing any adverse results.

The solvent used in the practice of this invention is a high boiling hydrocarbon having a high aromatic content. The solvent, to operate satisfactorily, contains at least about 50 wt. percent aromatics and has a minimum initial boiling point at atmospheric pressure of about 350F, and preferably about 400F, and a minimum upper limit of the boiling range (end point temperature) of at least about 500F, preferably 600F minimum. The solvent will have a Kauri-butanol value of at least about and preferably from about to about 1 10. The Kauri-butanol test is a well-known standard test for solvents. The preferred solvents falling within the above specifications, are the heavy aromatic naphthas containing from about 75 to about 98 wt. percent aromatic constituents. If desired, or if necessary for thorough cleaning, a minor amount of chlorinated benzenes, particularly ortho-dichlorobenzene, can be used conjointly with the above described solvents to clean the heat exchanger, Generally, the chlorinated benzene will be used in an amount of about 10 percent by volume of the entire solvent, but the amount may vary within the preferable range of from about 5 to about 25 percent by volume. However, it is necessary when using a chlorinated hydrocarbon solvent in combination with the high aromatic solvent in the practice of this invention to carefully determine whether or not such chlorinated hydrocarbons can be allowed to contaminate the stream which passes through the side of the heat exchanger being cleaned, since chlorinated hydrocarbons are known to contaminate certain catalysts.

For purposes of illustration of the practice of this invention, reference is hereby made to the accompanying drawing wherein the heat exchanger E represents a shell and tube heat exchanger typical of those which are advantageously used in hydrocarbon processing plants and, therefore, are particularly adaptable to the use of practice of this invention. For purposes of this illustration, the tube side of the exchanger will be cleaned without interrupting the flow of the process stream through the shell side. From the following description, one of ordinary skill in the art will readily be able to accomplish the cleaning of the shell side of the exchanger by merely applying the stated procedure to the shell side process stream. During operation on the tube side, the process stream flows through line 10, valve 12 and thence through line 14 into the heat exchanger E. A bleed valve 16 is connected to line 14 between valve 12 and the heat exchanger E. On the exit side of the heat exchanger E, the material flowing through the tube side, whether it be heated or cooled, exits the exchanger through line 18, valve 20 and back to the process through line 22. Connected to line 18 is a bleeder valve 24. On the shell side of the heat exchanger E the process flow enters through line 26,

' exchanger E exits through line 34, valve 36 and returns to the process through line 38. To line 34 is attached bleeder valves 24 and 16 on the tube side and 40 and 32 on the shell side and, using proper safety precautions for the material and temperatures and pressures involved, drain the exchanger. The exchanger can then a bleeder valve 40. When cleaning the tube side of the be P P Y COOlEd y circulating i g thIOUgh heat exchanger, a line 42 is connected to bleeder valve both Sides Ofthe heat exchahger- Again for P P of 16 thfgugh an ptional heck alve 44 The inlet Valve illustration, the tube Side is used and the aromatic 12 is closed, the bleeder valve 16 opened and the solsolvent is Pumped thmugh the bleed Valve 16 ihto the vent pumped b pump p through line 42 the Optional exchanger E with venting through the outlet bleed check valve 44 and thence through the bleeder valve Valve Ohce the exchanger full of Solvent, the 16, line' 14 and into the exchanger E t replace h bleed valves are closed and the outlet valve opened cess material which is wi hi h exchanger, Th to allow the tube side of the heat exchanger to again amount of solvent pumped is sufficient to fill the exfl n h p n Outlet valve. h pr r m. changer and force all of the process mat ial t normally flowing through the shell side, will then be althrough the exit valve 20 which remains open d in 15 lowed to enter the shell side by opening valve 28 using the cleaning process of this invention. Once the exthe normal safety precautions for starting up ashell and changer is filled, the process material flowing through tube heat exchanger where differential pressures are the shell side of the heat exchanger E heats the solvent involved. These procedures are ell-known to those in contact with the carbonaceous fouling material and skilled in the art. The process stream will then flow causes said material to be removed from the heat ex- 0 through the shell side of the heat exchanger E with change surfaces. This cooking process is allowed to valve 36 having been opened, the'bleeder valves, of continue for at least about 8 and preferably'at least course, have been closed. about 18 hours with the time generally ranging from To return the heat exchangerEto normal process opabout 8 to about 24 hours, preferably from about 18 to eration, the procedure discussed above is to be folabout 24 hours, depending upon the nature of the car- 25 lo ed. bonaceous fouling. Longer times can be used but as a Those of ordinary skill in the art, after having read practical matter are rarely necessary and generally conthe above discussion of the process of this invention, sidered wasteful. will have no trouble applying it to other situations Once the cleaning has been completed, to return the wherein shell and tube heat exchangers are used in bytube side of the heat exchanger to on-stream operation, drocarbon processing. assuming. for example, the heat exchanger E is a part The following Examples are offered by way ofillus' of the crude preheat stream where the heavy aromatic tration of the practice of this invention and should not solvent would not contaminate or harm the feed be construed as limiting the scope thereof. Many modistream; it is only necessary to open the inlet valve 12 fications and variations thereof will be obvious to one with the bleed valve 16 being closed to allow the proof ordinary skill in the art after studying the foregoing cess fluid,crude in this case, to push the heavy arodiscussion and considering the following Examples. matic solvent out through the exit valve 20 and thence into the process stream. Thus, it is seen from the fore- EXAMPLES 1 4 going description that the heat exchanger has been off line only a very short period of time, as compared with 40 In the following Examples, the heat exchangers prior art processes for such heat exchanger cleaning. lt cleaned were in the crude preheat train of a refinery has not even been necessary to discontinue the flow of with incoming crude on one side and hot product from the material on the shell side of the exchanger. the crude fractionation tower on the other supplying lf it is desired to avoid pushing the solvent into the the heat to cook the solvent. The solvent used was a process stream, once cleaning is completed, exit valve heavy aromatic naphtha containing 98 wt. percent aro- 20 can be closed and bleed valve 24 opened, along with matics and having a boiling range of 405F to 690F inlet valve 12 and the solvent piped away to a remote and a Kauri-butanol value of 105. The inlet crude valve location in the plant using proper safety precautions in was closed and the crude wasdisplaced from the heat view of the high temperatures generally involved and exchanger by solvent pumped with a positive displacethe danger of fire which inheres in the handling of hyment pump through the inlet bleed valve on the side of drocarbon materials at high temperatures and presthe exchanger being cleaned. After a volume sufficient sures. to fill the side of the exchanger being cleaned had been The use of the method of this invention is adaptable pumped, the positive displacement pump was cut off to process parameters involved in operating the particis and the inlet bleed valve was closed. The exit valve on ular shell and tube heat exchanger involved and such the side of the exchanger being cleaned was left open process parameters are not critical to the practice of and the heat exchanger was allowed to float on the this invention, but should be kept in mind by the skilled open valve during the time of the cleaning. After the engineer in order that proper safety precautions be folcleaning was complete, the inlet crude valve was lowed at all times. opened and the solvent, with removed carbonaceous If desired, the above described cleaning process material, entered the crude stream through the exit could be carried out by first taking the heat exchanger valve of the exchanger. The pertinent information con- E out of service by closing valves 12, 20, 28 and 36 to cerning the cleaning and the results thereof are set stop flow of process material in the exchanger and open forth on Table 1 below.

TABLE 1 Heat Transfer Pressure Increased Side Volume Temp. Time. Coefficient Drop, psi Throughput Ex. Cleaned Gals. "F hr. Before After Design Before After bbl/day 1 Tube v500 780 18 12.5 24.4 33 56 2.500

TABLE 1 Continued Heat Transfer* Pressure lncreased Side Volume Tempf" Time, Coefficient Dropf" psi Throughput Ex. Cleaned Gals. F hr. Before After Design Before After bbl/day 2 Shell 1.000 550 24 28.0 101 79 N/A N/A 6,000 3 Tube 750 350 18 25.5 63.6 75 N/A N/A 4.000 4 Shell 1.200 785 18 21.0 61.0 33 102 41 6,000

U, BTU/(hr)(ft.)(F) for side cleaned for side cleaned process side It was estimated that as a result of the cleaning described in Example 1 a savings of $10,000.00 per month resulted from reduced fuel consumption in the crude oil furnace in the system.

EXAMPLE 5 Two exchangers in series in a crude preheat train with hot product from the crude fractionation tower on the tube side and incoming crude on the shell side were cleaned with a solvent having 28 to 30 wt. percent aromatic content, although it fit the proper boiling range. The shell side ofthe exchangers were cleaned using the procedure described in Examples 1-4 for 24 hours. The first exchanger had a shell side heat exchange coeffieient, U,,, of 16.5 before cleaning and 18.6 after cleaning. The second exchanger had a U of 24.5 before cleaning and a U of 26.0 after cleaning. Tube side temperature was 476F. The amount ofcleaning was unsatisfactory.

A second cleaning of these two exchangers using the same process for the same length of time was carried out using the solvent described in Examples l-4. This cleaning increased the U, on the first exchanger to 32.8 and the second exchanger to 34.4. Design U,, on the first exchanger is 42.0 and on the second 44.0. The crude throughput increase was estimated at 8.000 bbl/day.

EXAMPLE 6 ln a crude preheat train of a refinery a heat exchanger with the incoming crude on the shell side was cleaned in the manner described in Examples l-4. The solvent used was 10 percent by volume orthodichlorobenzene and 90 percent by volume of the heavy aromatic naphtha of Example 1. The cleaning took place for a period of 18 hours with a process side temperature of 450F. The volume ofthe shell side was 1,200 gallons and the flow through said shell side was increased by 2,500 barrels per day after cleaning. The heat transfer coefficient. U,,, for the shell side before cleaning was 24 and after cleaning 67 with a design heat transfer coefficient of 72.

In view of the foregoing description, drawing and Examples, one of ordinary skill in the art may make many modifications and variations of the above-described invention without departing from the scope of the claims attached hereto.

We claim:

1. A method for removing carbonaceous fouling from shell and tube heat exchangers used in hydrocarbon processes which comprise the steps of:

a. displacing a process stream on one side of the heat exchanger with a hydrocarbon solvent having at least about 50 wt. percent aromatic content, a Kauri-butanol value of at least about 90 and a minimum initial boiling point of at least about 350F and an end point temperature of at least about 500F, and retaining said solvent in said one side of the heat exchanger;

b. heating the solvent being retained in one side of the exchanger by passing a process stream through the other side of the heat exchanger as innormal operation of the heat exchanger for a period of at least about 8 hours to remove the carbonaceous fouling; and

c. displacing the solvent and removed fouling with the process stream.

2. The method of claim 1 wherein the solvent and removed fouling are displaced into the process stream itself.

3. The method of claim 1 wherein the flow of the process stream on the side not being cleaned remains uninterrupted during step (a).

4. The method of claim 1 wherein the heat exchanger is taken out of service and cooled prior to step (a).

5. The method of claim 1 wherein the solvent includes about 10 volume percent of a chlorinated benzene.

6. The method of claim 5 wherein the solvent and removed fouling displaced by the process stream is removed from the process.

7. The method of claim 1 wherein the solvent is a heavy aromatic naphtha.

8. The method of claim 7 wherein the solvent has an aromatic content of from about wt. percent to 98 wt. percent and a boiling range of from about 400F minimum initial boiling point to about 600F minimum end point and a Kauri-butanol value of from about 100 to about 1 l0.

9. The method of claim 8 wherein the cleaning was continued for a period of from about 18 to about 24 hours.

10. A method for removing carbonaceous fouling on the side of a shell and tube heat exchanger carrying crude oil in the crude preheat train of a petroleum refinery which comprises the steps of:

a. displacing the crude in the heat exchanger with a heavy aromatic naphtha solvent having an aromatic content of from about 75 wt. percent to 98 wt. percent a Kauri-butanol value of about and a boiling range of from about 405F to about 690F without interrupting process flow on the other side of said heat exchanger;

. heating the heavy aromatic naphtha being retained on the crude oil side of the exchanger by passing the hot process stream through the other side of said exchanger as in normal operation of the heat exchanger for a period of from about 18 to about 24 hours to remove the carbonaceous fouling; and

c. displacing the solvent and removed fouling by flushing the solvent and removed fouling into the crude stream flow with said crude oil.

11. The method ofclaim 10 wherein the exit valve of the crude oil side remains open during heating of the solvent being retained therein. 

1. A METHOD FOR REMOVING CARBONACEOUS FOULING FROM SHELL AND TUBE HEAT EXCHANGERS USED IN HYDROCARBON PROCESSES WHICH COMPRISE THE STEPS OF: A. DISPLACING A PROCESS STREAM ON ONE SIDE OF THE HEAT EXCHANGER WITH A HYDROCARBON SOLVENT HAVING AT LEAST ABOUT 50 WT. PERCENT AROMATIC CONTENT, A KAURI-BUTANOL VALUE OF AT LEAST ABOUT 90 AND A MINIMUM INTIAL BOILING POINT OF AT LEAST ABOUT 350*F AND AN END POINT TEMPERATURE OF AT LEAST ABOUT 500*F, AND RETAINING SAID SOLVENT IN SAID ONE SIDE OF THE HEAT EXCHANGER; B. HEATING THE SOLVENT BEING RETAINED IN ONE SIDE OF THE EXCHANGER BY PASSING A PROCESS STREAM THROUGH THE OTHER SIDE OF THE HEAT EXCHANGER AS IN NORMAL OPERATION OF THE HEAT EXCHANGER FOR A PERIOD OF AT LEAST ABOUT 8 HOURS TO REMOVE THE CARBONACEOUS FOULING; AND C. DISPLACING THE SOLVENT AND REMOVED FOULING WITH PROCESS STREAM.
 2. The method of claim 1 wherein the solvent and removed fouling are displaced into the process stream itself.
 3. The method of claim 1 wherein the flow of the process stream on the side not being cleaned remains uninterrupted during step (a).
 4. The method of claim 1 wherein the heat exchanger is taken out of service and cooled prior to step (a).
 5. The method of claim 1 wherein the solvent includes about 10 volume percent of a chlorinated benzene.
 6. The method of claim 5 wherein the solvent and removed fouling displaced by the process stream is removed from the process.
 7. The method of claim 1 wherein the solvent is a heavy aromatic naphtha.
 8. The method of claim 7 wherein the solvent has an aromatic content of from about 75 wt. percent to 98 wt. percent and a boiling range of from about 400*F minimum initial boiling point to about 600*F minimum end point and a Kauri-butanol value of from about 100 to about
 110. 9. The method of claim 8 wherein the cleaning was continued for a period of from about 18 to about 24 hours.
 10. A method for removing carbonaceous fouling on the side of a shell and tube heat exchanger carrying crude oil in the crude preheat train of a petroleum refinery which comprises the steps of: a. displacing the crude in the heat exchanger with a heavy aromatic naphtha solvent having an aromatic content of from about 75 wt. percent to 98 wt. percent, a Kauri-butanol value of about 105 and a boiling range of from about 405*F to about 690*F without interrupting process flow on the other side of said heat exchanger; b. heating the heavy aromatic naphtha being retained on the crude oil side of the exchanger by passing the hot process stream through the other side of said exchanger as in normal operation of the heat exchanger for a period of from about 18 to about 24 hours to remove the carbonaceous fouling; and c. displacing the solvent and removed fouling by flushing the solvent and removed fouling into the crude stream flow with said crude oil.
 11. The method of claim 10 wherein the exit valve of the crude oil side remains open during heating of the solvent being retained therein. 